The R-Pi has fuses on the power output through the two USB ports limiting the current on each one to about 140 mA. Since the R-Pi needs about 550 to 700 mA when running at full tilt, the system will lock up if you try to power it only by back-feeding current through the USB output. You can prevent the backfeeding by taping over the +5V pin on the USB connector going to the powered hub before connecting it. But you may not need to, it should not cause a problem so long as you have +5V power normally supplied, and yes you can supply it from that same powered hub using a microUSB cable. As long as the hub can provide enough current!